1. Field of the Invention
This invention relates to a color-developing sheet having less deterioration of a color-developing ability with lapse of time and superior plasticizer resistance.
2. Prior Art
Pressure-sensitive recording sheets are known as carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies. A colored image is formed by a color-developing reaction between an electron-donating colorless dye and a electron-accepting color-developing agent.
Hitherto, many kinds of electron-accepting color-developing agents are well-known. The typical color-developing agents include inorganic solid acids such as acid clay, activated clay, attapulgite (described in U.S. Pat. No. 2,712,507); substituted phenols and diphenols (described in the Japanese Patent Publication No. 9309/1985); p-substituted phenol-formaldehyde polymers (described in the Japanese Patent Publication No. 20144/1967); aromatic carboxylic acid metal salts (described in the Japanese Patent Publication Nos. 10856/1974 and 1327/1977); 2,2'-bisphenol sulfone compounds (described in the Japanese Patent Laid-Open No. 106313/1979); and so on. Among above color-developing agents, phenolic polymers, such as p-substitute phenol-aldehyde polymers, etc., are widely applied for practical use since they are produced inexpensively and provide sheets having a high image density and a superior resistance of an image under the action of light, humidity and heat.
However, the color-developing sheet containing a phenolic polymer as a color-developing agent has following defects: a color-developing rate and an end color-developing intensity deteriorate in storage (hereinafter referred to as "the deterioration of color-developing ability with the lapse of time"; the stability of colored image on the contact with oils, particularly plasticizer, (hereinafter referred to plasticizer resistance) is inferior; and the like.